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Hannu Savijärvi

Abstract

The Euler-backward time integration method is discussed and it is suggested that a second backward iteration could be advantageous, allowing strong damping of short waves, very small phase error and long time step at the expense of increased computations.

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Hannu Savijarvi

Abstract

Internal and external z500 global total rms errors followed quadratic growth laws quite well in NMC Medium-Range Forecast (MRF) Model 0–10-day forecasts for 1988–93. Growth parameters and model and analysis errors for many winter were estimated using the quadratic rms error growth assumption. Both the MRF model error and analysis error have nearly halved during 1988–93. But at the same time the growth parameters have nearly doubled: smaller errors grow faster. Thus while the limit of deterministic predictability (rms error 71% of saturation) has been going up, the limit of dynamic predictability (rms error 97.5% of saturation) seems to be set at around 20 days in large horizontal scales, dropping to 6–7 days in small scales.

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Hannu Savijärvi

Abstract

Global ECMWF data for winter and summer seasons are used to study the spectral structure of 500 mb analyzed height variance and forecast error variance, as well as the effect of spectral truncation in the height and temperature variance calculations. The high wavenumber region of the variance of the analyses is nearly homogeneous and isotropic and follows the −5 power law both in the zonal, two-dimensional and meridional wavenumber indexes. The high wavenumber region is dominated by transient waves. The low wavenumber region is dominated by the stationary part, being anisotropic and seasonally variable. The transition between the two regions is at wavenumbers 7–8. The temperature variance follows a −4 power law in the high wavenumber region.

The ECMWF forecast error variance is dominated by the nonsystematic error, the maximum of which shifts from medium to long zonal waves during the forecast. The nonsystematic error variance spectrum is quite similar to the transient variance in the analyses.

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Hannu Savijärvi

Abstract

Observations made from an Oklahoma City TV tower are first used to test the one- and two-dimensional versions of a numerical mesoscale model. Both model versions produce a fairly good reproduction of the observed typical diurnal variation in the wind, temperature, and turbulent mixing coefficient profiles.

Next, the models are used to study slope winds and the nocturnal low-level jet (NLLJ) formation, starting with the Prandtl slope wind theory conditions and eventually adding more complex physics (e.g., relating to slope angle, mixing, radiation, background flow, and heat input conditions). The models are able to reproduce the Prandtl analytical solution in the appropriate simple conditions. The time scale to reach this solution from an initial rest state is, however, very long for gently sloping plains.

Results with the full model indicate that the main mechanism for the Great Plains “average” NLLJ is the inertial oscillation due to frictional decoupling after sunset. Under normal conditions the sloping of the plains has only a small guiding effect. Direct radiative solar heating and longwave cooling of the atmosphere also have only a small effect, even at nighttime, as mechanical turbulent mixing is efficient during typical NLLJ flow.

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Hannu Savijärvi

Abstract

Three fast and simple longwave broadband radiation parameterization schemes were tested against a reference narrow-band model in clear sky conditions. The three schemes gave rather similar results. The emissivity dependence on water vapor path length was tuned to give best fit to the reference. The smaller other gas, aerosol and continuum circus were added in a simple fashion both to the radiative cooling and to the longwave downward flux at the surface. Clouds are handled as blackbodies.

In the shortwave scheme the attenuation of solar radiation due to variable water vapor and gray clouds, but average ozone and other gas absorption, aerosols and Rayleigh scattering are taken into account and tuned by surface observations of solar flux. The water vapor absorptivity for solar heating was fitted by line-by-line model results.

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Hannu Savijärvi and Tero Siili

Abstract

The summertime Martian PBL diurnal wind variation, slope winds, and the nocturnal low-level jets were studied using Prandtl's theory, a mesoscale numerical model, and Viking lander observations. During moderate prevailing large-scale flow, nocturnal jets were simulated that were rather similar to those on Earth. They were mainly caused by inertial oscillation after sunset with some contribution from the slope wind effects over sloping regions (which are very common in Mars). During weak large-scale flow, shallow nocturnal drainage flows with strong vertical shear developed over the cold Martian slopes. At middle and high latitudes, these katabatic winds tended to turn to flow along the slope by dawn (due to the Coriolis force). For sufficiently steep slopes, near-surface drainage winds could reach considerable speeds. In contrast, the typical afternoon upslope winds were vertically homogeneous up to 2–3 km and weak (only 1–3 m s−1 in magnitude), even over relatively steep large-scale slopes.

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Hannu I. Savijärvi

Abstract

The budget equations for the transient variance of a scalar s (s2¯) and its squared time mean (s 2¯) are derived for a hydrostatic fluid. They are discussed and illustrated with two alternative box diagrams. The 10 yr rawinsonde global data of Oort are used in presenting and discussing the observed atmospheric variance budgets of potential temperature and moisture in the DJF season, excluding year-to-year variations. The results indicate that the daily variances (θ′2¯ and q2¯) are maintained largely by the downgradient heat and moisture horizontal transient eddy (TE) fluxes which are strong in the midlatitude baroclinic zones (storm tracks) in both hemispheres. The flux convergence (advection of existing variance by the time-mean wind) is small and so the production must be compensated by the damping net effect of upgradient vertical potential temperature fluxes, third order flux divergences and time correlations between eddies and forcing. In contrast, the budgets of the squared means (θ′2¯ and q 2¯) are governed by the time-mean sources with compensating large advections by the mean wind and a smaller smoothing effeect from the transient eddies.

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Hannu Savijärvi and Stuart Matthews

Abstract

A two-dimensional nonlinear model with physical parameterizations was applied to simulate the observed diurnal variation on the 5-km-wide flat tropical island of Nauru in the trade wind zone. Both the model and Atmospheric Radiation Measurement (ARM) campaign aircraft observations indicate vigorous mixing in the typical sunny daytime conditions, leading to a warm plume downstream of the island. The model's afternoon wind field displayed rising motion downstream and downwash ahead of the island with gravity wave structure, in accordance with linear models of steady flow over a heated island. The roughness difference between sea and land added local rising motion above the windward coast and sinking motion above the lee.

Without large-scale wind U, a weakish sea-breeze (SB) pattern develops during the day in this model over the 5-km-wide island/peninsula. This pure SB circulation intensifies with increasing island width up to 40 km. When large-scale wind is present, the morning leeside SB cell is advected out to sea and disappears while the windward coast SB cell tilts over the island and is transformed into the steady heat island–type perturbation during the day.

During the night, a reversed heat island–type weak and shallow perturbation develops for nonzero U. ForU = 0, the sea breeze dies in the evening and no land breeze appears. If a 200-m-high central mountain is added to a 20-km-wide island/peninsula in calm daytime conditions, the SB circulation is enhanced by upslope winds followed by weak katabatic flow down the cool slopes during the night. When any large-scale flow is present, the forced flow up and down the slopes appears to dominate the wind perturbation patterns day and night.

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